The invention relates generally to implantable medical devices and their methods of use for stabilizing skeletal bone, and relates more particularly to implantable medical devices fabricated of metals and their use for stabilizing the vertebrae of a human spine.
With normal anatomy, the vertebrae of the spinal column are held together and to the skeleton by a complex arrangement of ligaments, tendons and muscles. Degenerative diseases, deformities, or trauma may cause abnormal conditions. These problems generally cause or allow displacement or rotation of a vertebra relative to the adjacent vertebra. When spinal discs rupture or bulge the intervertebral space between two adjacent vertebras 31 and 32 can decrease and cause discomfort to the patient. Frequently the bulging does no harm, but if it compresses against the spinal cord or a nerve it may cause pain, loss of sensation, or weakness. When surgery is needed, the discs may be replaced with grafts that will heal or xe2x80x9cfusexe2x80x9d the adjacent vertebrae together. This implant, with its associated stabilization, maintains the vertebral position while healing takes place. This is referred to as xe2x80x9cspinal fusionxe2x80x9d. The objective of spinal implants is to facilitate realignment and/or fixation of spinal elements. Clinical studies have demonstrated that surgeries using spinal implants are more effective at maintaining alignment and providing rigidity to the spine than surgeries in which implants are not used. Since the introduction of stabilizers as crude plates, rods, and wires, these devices have been developed into sophisticated appliances, which can be assembled and configured to rigidize spines of any size or condition. These stabilizers provide mechanical fixation for restraint of an implanted graft material. With this fixation, displacement during healing is significantly reduced thereby reducing the failure rate of the fusion surgery.
Prior Technology
The majority of existing spine stabilizers use plates that are bent in both the axial plane to conform to the vertebrae, and along the spinal axes to maintain lordosis. Bicortical screw purchase has been favored because of the increased strength of the construct and increased screw thread area within the bone. These screws are more technically challenging to place and add increased risk of morbidity from neural canal penetration and screw backout. The reduced strength and decreased thread area of a unicortical screw purchase increases the probability of screw back out or loosening resulting in soft tissue organ (e.g. esophageal) injury and loss of rigidization of the fusion construct. Screw back out and loosening has led to the development of mechanisms for locking the screw head to the plate in unicortical screw plate designs. Such locking mechanisms not only prevent screw back out they also reduce the tendency of the screw head to pivot within the plate. These devices contain many intricate components that increase the cost and reduce reliability. The unicortical devices presently available are relatively rigid devices.
U.S. Pat. No. 5,578,034 to Estes discloses a bone screw with an enlarged head and an annular collar surrounding the bone screw shaft. The collar""s inner diameter shrinks in response to a change in temperature, trapping the collar between head and the threads of the bone screw.
U.S. Pat. No. 6,269,716 to Amis discloses a tapered screw head for biodegradable medical implants. The screw head has a star shaped outer circumference with external features for rotation. In the disclosed patent the resorbable fastener tapered head is connected to a threaded shaft. The stress raisers of both the threaded portion and tapered head are in the high stressed area at the plate/bone interface. This design is successfully used in non-load bearing bones in facial and cranial surgeries. However it does not have the required strength for load bearing applications.
U.S. Pat. No. 5,549,612 to Yapp discloses an osteosynthesis plate with a locking mechanism that is integral with the plate comprising a rotatable cam adjacent to the screw holes to inhibit movement of the screw.
The following patent is an example of stabilizing systems that disclose or claim tapered screws:
U.S. Pat. No. 6,228,085 to Theken discloses a metallic bone fixation system with a three-dimensionally anatomically contoured plate to fit the anterior lateral profile of the vertebrae and forming a ledge to maintain the space between two vertebrae. The system is designed for use as a metal plate and is suited for thoracic and lumbar spines. It uses setscrews and threads in a portion of the hole. It has irregular surfaces in the plate such as steps, spines or teeth to bite into a bone. Such features are costly. The screw may have a tapered outer surface adjacent to the threaded portion, to provide pullout resistance of the screw in the plate.
Locking Tapers
Locking tapers, sometimes called self locking or self-holding tapers, are tapered round shanks that fit into round sockets with matching taper angles. These tapers are usually less than 5 degrees on a side. During engagement the shanks are firmly seated in the socket by an axial force such as tapping with a hammer or drawing in with a screw thread. These axial forces provide a normal force component that is sufficient to create frictional forces, which will resist relative rotation of the shank with the socket when they are assembled.
Interference Fits
Interference fits between two parts are created by forming the parts so that one or both contacting surfaces yield to create high normal compression forces. These forces provide friction to retain one piece (the male part) in the other piece (the female part) to decrease or eliminate one piece from moving with respect to the other under axial or rotational loading. In the case of cylindrical shafts and cylinder holes, the shaft diameter may be formed slightly larger than the hole diameter. This type of assembly is also called a press fit. The amount of interference is controlled during manufacture and cannot be altered during assembly. If the shaft and hole are manufactured of matching tapers, the assembly force and displacement may control the amount of interference.
The preferred metals for implants are Titanium alloys, especially TiA16V4, because of the decreased interference with magnetic resonant imaging (MRI) techniques used for postoperative evaluation. This is due to minimal ferromagnetic properties which also improve lifestyle considerations when metal detectors come in play. TiA16V4 is a well documented material in clinical use with an excellent safety record and FDA approval.
The present patent discloses a method and a device for stabilizing vertebrae in a human spine for the purpose of temporarily fixing the vertebra with respect to other vertebrae and with respect to other parts of the spinal column. This device comprises a metallic plate and bone screws fabricated from metals. The plate has a plurality of tapered holes with the smaller diameter end adjacent to the vertebra and the larger diameter on the opposite side of the plate. The bone screw has a threaded portion that engages a predrilled and/or threaded hole in the vertebra or the graft. The bone screw also has a tapered portion with a major diameter greater than the large diameter of the tapered hole. The bone screw maintains the plate in contact with the vertebra. The screw tapered portion is pulled into a matching tapered hole, locking the screw to the plate. The taper is configured to be self-locking, preventing the screw from backing out.
An object of the present invention is to provide a method and a device for a fusion, fixation and/or for spinal stabilization.
Another object of the present invention is to provide a reliable stabilizer device which is effective yet uncomplicated mechanically and simple to manufacture.
Another object of the present invention is to provide a spinal fusion and a spinal stabilization device using metallic stabilization plates and plate attachment devices.
Another object of the present invention is to provide devices and methods for cervical, thoracic, and lumbar spinal fusions anteriorly, posteriorly, and/or laterally.